A. Field of the Invention
This invention relates to the can manufacturing art, and more particularly to a novel construction and arrangement of the bottom portion of a drawn and ironed can body and method for making such a can body.
B. Description of Related Art
It is well known to draw and iron a sheet metal blank to make a thin-walled can body for packaging beverages, such as beer, fruit juice or carbonated beverages. In a typical manufacturing method for making a drawn and ironed can body, a circular disk or blank is cut from a sheet of light gauge metal (such as aluminum). The blank is then drawn into a shallow cup using a cup forming equipment. The cup is then transferred to a body maker where the can shape is formed. The body maker re-draws and irons the sidewall of the cup to approximately the desired height, and forms dome and other features on the bottom of the can. The dome and other feature on the bottom edge of the can are referred to herein as the “bottom profile” of a drawn and ironed can body.
Can body manufacturing techniques are described in the patent literature. Representative patents include U.S. Pat. Nos. 6,305,210; 6,132,155; 6,079,244; 5,984,604, and 5,934,127, the contents of which are incorporated by reference herein. Domer assemblies for drawing and ironing machines are described in U.S. Pat. Nos. 4,179,909; 4,620,434; 4,298,014, all assigned to National Can Corporation, the contents of which are incorporated by reference herein.
In current practice, after the can is formed in the body maker, the can is sent to a separate necking and flanging station, where the neck and flange features are formed on the upper regions of the can. The flange is used as an attachment feature for permitting the lid for the can, known as an “end” in the art, to be seamed to the can. The last station in the necker-flanger is a reformer station. This station includes a set of tools for reforming the bottom profile of the can in order to increase the strength of the bottom profile. U.S. Pat. Nos. 5,222,385 and 5,697,242, both assigned to American National Can Co., describe a can body reforming apparatus and methods for reforming can bodies to increase the strength of the bottom profile. Ihly, U.S. Pat. No. 5,934,127 also describes can bottom reforming apparatus. Other patents of interest include Gouillard, U.S. Pat. No. 6,132,155 and Saunders et al., U.S. Pat. No. 6,305,210. After necking, flanging and bottom reforming, the top edge of the can is trimmed.
Long ago, when cans were made from a relatively heavy gauge aluminum, a bottom profile could be formed by the body maker that did not require a separate reforming operation in order to increase the strength of the bottom of the can. The separate reforming operation was not necessary due to the relatively thick aluminum gauge material providing the required strength. However, under current practice, aluminum stock used for drawn and ironed beverage cans is of a much thinner gauge than it used to be, in order to reduce the amount of material used to make a can. Consequently, it is much more difficult to provide a can bottom profile resulting from the shaping performed by the body maker that possesses the strength required to meet customer requirements for bottom performance. Thus, in accordance with the present practice of the assignee of this invention, after formation of the can by the body maker, the separate bottom reforming step is performed to further form or shape the bottom of the can in order to increase the strength of the bottom profile and allow it to meet customer requirements in terms of can bottom performance.
The bottom performance of a drawn and ironed can body is typically characterized by three independent and distinct criteria: can growth, drop, and buckle. Can growth refers to a deformation of the can bottom due to the pressurized contents of the can causing the bottom of the can to extend further in the axial direction. The can is pressurized to 90 PSI, the pressure is removed, and the growth g is measured. The phenomenon is shown in FIG. 1. The can bottom profile prior to pressurization is shown in dashed lines, the bottom profile after pressurization is shown in solid lines. The bottom profile 10 includes a nose portion 12 which defines a circumferential stand or base on which the can sits. The bottom profile 10′ after growth shows the nose portion 12′. Growth occurs by an unrolling action of the nose 12, wherein the material forming the nose moves away from the region of the dome 14. Growth resistance is thus a measure of the stiffness of the bottom profile—how much pressure can the can withstand before the nose 12 unrolls, and the amount g of can growth at a given pressure. As is known in the art, the tighter the radius of the nose 12 is, the more pressure required to “unroll” the nose and incur can growth. Hence, bottom profile reforming typically involves reforming the nose so as to decrease the nose radius to improve can growth characteristics.
Drop refers to a measurement of the height at which a can, filled with water and pressurized with nitrogen to 60 pounds per square inch, is dropped and lands square on a steel platform, which results in a reversal (either whole or partial) of the dome in the bottom of the can, such that the can will no longer stand without tipping. The drop height starts at three inches and increases one inch until the failure criteria is reached. Typically, 10 or more cans are tested and the average and standard deviation are reported as results. During a drop test, the sudden dynamic load of the liquid increases the pressure on the dome. The result is shown in FIG. 2. The figure shows the dome 14′ (solid line) just prior to the dome reversal. The dome at 14′ in the Figure is not the final shape of the dome at failure, as in the final configuration the dome completely reverses, as shown. The following results are observed, as shown in FIG. 2: The nose is restrained from unrolling (as shown in FIG. 1) by the steel platform; the inner leg or chime 16 rotates outwardly and results in a negative angle; a more shallow dome results; and the dynamic load of the liquid in the can causes a local collapse of the dome 14. The dome becoming shallower does not constitute a failure; the inability to stand a can without tipping is considered a failure.
Buckle refers to the internal pressure limit (e.g., 100 PSI) at which point the dome in the bottom of the can reverses. Like the growth issue described previously, dome reversal involves a dynamic “rolling” at the nose of the can. See FIG. 3. Dome reversal occurs when there is no more leg material available to the roll (nose), additionally the leg angle tilts inwardly by a considerable margin (positive angle). A design goals for increasing buckle is to provide a deeper dome depth, reforming to tilt the leg angle outward (provide a negative chime angle) and provide a larger nose radius and dome corner radius to provide more material for the dynamic rolling of the nose.
As is known in the art, and as indicated by the above discussion, changing the parameters or values of the various features of the can bottom profile (dome radius of curvature, stand diameter, nose radius, chime angle, etc.) tend to effect the ability of the can to meet the above-referenced bottom performance criteria. However, a change in a particular value in the can bottom profile may result in a positive improvement in one criteria (such as minimize can growth), but at the same time negatively affect one or more of the other parameters (such as, for example, lower the buckle limit and lower the drop limit). Complicating the situation is the fact that can bodies are made from a very thin gauge of aluminum material, and as the material becomes thinner, it becomes increasingly difficult to design the can body that meets all the bottom performance criteria.
Further considerations of the design of the bottom of a can are reduction in bottom wrinkling and reduction in bottom thinning. These considerations, in addition to the previously described goals of increasing bottom performance in terms of buckle, drop and growth, typically oppose each other. In other words, the steps a designer may take to improve can bottom performance may actual work against reducing bottom wrinkling or bottom thinning.
Accordingly, there has been a need in the art for a new and improved can body which optimizes the various can bottom design parameters such that it not only meets the bottom performance criteria required by the industry, using current gauge material for the can body, but allows the can body to be formed without requiring a separate reforming process to strengthen the bottom profile. This need is particularly strong in today's environment since the can body reforming process can represent the most consistent bottleneck in high-speed can manufacturing operations. It has been the experience of the inventors that the reforming tools require more frequent maintenance and are more prone to problems than the other equipment used in the process. Furthermore, to the extent that the bottom reformer can be completely eliminated, it represents a savings in capital expense, since the equipment does not have to be purchased, and savings of labor and energy consumption.
An objective of the present invention is to provide a bottom profile design for a thin walled drawn and ironed can body made from 0.01075 inch gauge material or thinner which does not require a separate reforming step in order for the can body to meet customer (industry) strength requirements for bottom performance, passes a drop test of at least 5½ inches, and has acceptable bottom wrinkling and bottom thinning characteristics.